All you have to do is be in the same light, then you use it similarly to the studio.
If you aren't in the same light sometimes walking a few steps can get you there.
You can also do non-traditional measurements. If you are in shadow you will get a reading that will give you the equivalent of shooting at 1/2 or 1/4 box speed, for example. You can shoot there or adjust to correct, your choice.
BTZS is actually a zone system variant that uses incident readings normally, even preferentially for exactly what you are asking to do. I'm not suggesting you have to do that, just an example.